How should i approach game asset design as a beginner?
That's quite a few questions and a bit more general than SE is meant for; but I'll tell you what I can.

Sculpting vs. box or face modeling really depends on the asset and what you're comfortable with.

Try and keep your vertices to a minimum necessary for the model, as every one is a tick against available resources on the client machine. You might also consider learning LOD, as you may end up using several different models (of leveled vertex count) for the in-game object depending on how far away from the camera it is.

Where texturing and optimization goes, it really depends on your game engine and isn't something I would worry about until you've profiled your game. Most of them do their own materials management and whatever you do in Blender you'll likely have to re-do there. They also accept a variety of formats, the most common being Filmbox and Collada; these notoriously differ in what they're capable of and how they do it, so bear it in mind.

And yes, Blender 2.8 is easily enough on its own; but you might get some use out of additional software. It's a little too vague to properly answer here.

I hope that helps. I'm still going to suggest that this question be closed, as it's a bit broad and relatively opinion-based.